The Molecular Field and Atomic Order in Ferromagnetic Crystals and in Hydrogenised Iron

Abstract

THE molecular field postulated by Weiss to explain ferromagnetism is regarded by Becker¹ as mechanical in origin, representing the control exerted by strain within any portion of a crystal upon the direction in which that portion may retain its magnetisation in the absence of externally applied magnetic fields. More recently, Becker and Kirsten² have discussed some interesting experiments upon the magnetisation of nickel under tension. They arrive at the conclusion that, at least for applied fields much larger than the coercive force—which becomes as small as one or two gauss under their extreme conditions—the magnetisation may be predicted quantitatively from the maximal magnetostrictive shortening of well annealed metal and the saturation intensity of magnetisation, both of which quantities are sufficiently well known from previous experiments by others. Their account of the effect of internal stresses over the cross-section of a wire specimen upon its magnetisation is also very satisfactory. They point out, in the second footnote to p. 660, that the “extremely irreversible processes” observed in weak applied fields are not correctly represented by Becker's theory, magnetic changes being observed at field values which are less than the theory predicts as necessary for their occurrence.^*